Author:Samar Kalita
from
Engineered Surfaces Center, University of North Dakota
Douglas Larson
from
Engineered Surfaces Center, University of North Dakota
Ben Hoiland
from
Alion Science and Technology
Bryce Mitton
from
Engineered Surfaces Center, University of North Dakota
Juergen Fischer
from
Engineered Surfaces Center, University of North Dakota

Posted on:5/18/2011

It has been shown that the inexpensive chemically accelerated vibratory surface finishing (CAVSF) process can reduce the average surface roughness.

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Figure 1 - Large vibratory bowl with 1.16 m inner diameter and a dosing station in the background for continuous flow of chemicals through the bowl.

It has been shown that the inexpensive chemically accelerated vibratory surface finishing (CAVSF) process can reduce the average surface roughness of helicopter gear teeth from the conventional 16 μ-in. (0.39 μm) down to approximately 2 μ-in. (0.05 μm). Consequently the friction and the surface stress at the mating surfaces are substantially decreased, which results in a 300 to 400% increased fatigue lifetime, reduced downtime, less noise, higher energy efficiency, lower overall costs and reduced component weight if newly designed. The CAVSF process with different oxalic acid based solutions was studied using strip steel samples AISI 1018 in a 0.28 m diameter vibratory bowl. The effects of the chemical composition and concentration on the process were examined. The average end roughness and the material removal reached with the process were charted versus the pH of the different mixtures. The best results concerning average end roughness gave oxalic acid solutions with a pH around 1. At pH values higher than 3.0, the average end roughness starts increasing rapidly with pH. At a pH around 2.0, the material removal versus pH curve reaches a minimum and at a pH around 3.8, a maximum.

Introduction

Machining grind lines are a problem in critical working surfaces on gears, splines, journals, crankshafts, bearings, camshafts and couplings. These grind lines can have an average roughness of around 12 μ-in. (0.30 μm) and will impair lubricity. They cause vibration, friction, torque, higher operation temperature and noise. This leads to metal debris, plastic deformation, scuffing, wear and several forms of fatigue, which limit the useful life of equipment.

Throughout the world, the word superfinishing is often used for vibratory finishing, superhoning, stone finishing, tape finishing and microfinishing. Chemically accelerated vibratory surface finishing (CAVSF) is an isotropic surface finishing (ISF) process, which uses chemicals from a treatment solution to attack the surface and build a conversion layer. Ceramic or plastic media rubs off the conversion layer in a vibratory bowl. The CAVSF process is an environmentally friendly, inexpensive process to remove the machine grind lines, damaged material and asperities from the metal surface.1,2,3 It also reduces the stress risers.

The average surface roughness of case hardened steel surfaces on gear teeth decreases from around 12 μ-in. (0.30 μm) to around 2 μ-in. (0.05 μm) by using CAVSF. During the process, generally less than 200 μ-in. (5 μm) of material is removed. The geometry of the part is not impaired.

In this paper, the “removed material” is measured in μ-in (39.4 μ-in. = 1 μm). The removed material represents the average thickness of solid removed material across the whole geometrical surface area of the part. It is obvious, then, that the local removed material is higher on corners and edges and lower in cavities and recesses than this average. The weight loss and the specific density of the part are used for the calculation.

The average roughness Ra is measured with a profilometer that has a stylus with a 45° diamond cone. The tip of the cone has a radius of 1 μm or approximately 40 μ-in. The measured values for the average roughness are mostly between 0.5 and 30 μ-in. The diamond tip cannot measure the real average roughness which is about four times higher than the measured average roughness.4 The measured maximum roughness is about ten times higher than the measured average roughness.

It was found that the measured life improvement for superfinished gears was a factor of approximately five compared with conventional gears (average roughness: up to 16 μ-in. (0.41 μm) on the pitch line).5 A run-in time for superfinished surfaces is not required. Other benefits connected with the CAVSF process are reduced downtime, reduced component weight in new designs, less noise, less vibration, higher energy efficiency and lower overall costs.

This publication is part of the combined efforts of the Engineered Surfaces Center, which is part of the School of Engineering and Mines at the University of North Dakota, and Alion Science and Technology to validate, investigate, optimize and possibly improve the existing CAVSF process for critical surfaces. The report shows the results from the Engineered Surfaces Center concerning the investigation of the CAVSF process. Material removal and roughness changes during the process on different surfaces were measured, and process parameters were varied.

Test equipment and process steps

Two different vibratory bowls (Figs. 1 to 3) were available for testing the CAVSF process. The first has an inner diameter of 1.16 m, and the second has an inner diameter of 0.28 m.

During the process, a continuous flow of chemical solutions runs through the large vibratory bowl, which is filled with ceramic media, test pieces and additional steel parts. The vibratory motors run at 1800 rpm. The media and the test parts move in a toroidal helix around the bowl. Acid treatment solution is then sprayed into the bowl with a flow rate of 1.5 gal/hr (5.7 L/hr) for the first two hours, followed by a 15-minute water rinse cycle of 15 gal/hr (56.8 L/hr). Finally, 10 gal/hr (37.8 L/hr) burnishing solution runs through the vibratory bowl. The parts are taken out after approximately one hour of burnishing, rinsed in tap water and then in demineralized water, and dried with paper towels.

The small bowl (Fig. 3) runs without the continuous flow of chemicals and is filled with 2.2 L of ceramic media, four test pieces and a defined amount of treatment solution (hold-up). The acid treatment time is usually two hours as in the large bowl. It was found useful to run these quick tests with small amounts of different treatment solutions in the small bowl because fewer chemicals were wasted and cleaning and drying of the bowl and the media was easy. The roughness results were comparable but the material removal rate was only half. If needed the testing of the solution was repeated in the large vibratory bowl.

The influence of the pH in the chemically accelerated vibratory surface finishing (CAVSF) process with oxalic acid-based solutions

This paper will focus on the influence that the acidity, measured as pH, of some oxalic acid-based treatment solutions has on the material removal and the end roughness of the CAVSF process. It is the continuation of work published previously.6-9

Phase I: Proprietary and ammonium oxalate systems

In the first part of this work, two different acid treatment solutions were tested:

The tests were performed in a 0.28-meter diameter vibratory bowl with 3975 g of large, brown, ceramic media and four AISI 1018 strip steel samples. The average roughness of these samples was between 15 and 20 μ-in. at the beginning of the test. Every half hour a strip steel sample was removed, rinsed with demineralized water and dried with a paper towel. The roughness was measured and the weight loss determined to calculate the material removed in μ-in. After two hours of acid time the test was terminated.

Figures 4 and 5 show the material removal and the average roughness after two hours of acid treatment versus the pH of the acid treatment solutions. The “pH” is the pH of the solution added to the samples and the ceramic media, and is different than the pH that really matters in this case, the pH at the metal surface - beneath the conversion layer - where the reaction takes place. Because of the consumption of hydrogen cations in the area of reaction and the diffusion necessary through the conversion layer, the pH in the reaction zone will be higher than the measured starting pH.

Phase II: Oxalic acid / sodium oxalate systems

In the second part of this test series, different acid treatment solutions based on oxalic acid and sodium oxalate were tested (Table 3). Forty-seven milliliters of acid treatment solution made from various amounts of 0.2M oxalic acid and 0.2M sodium oxalate solution were added to 3745 g of white lens-shaped ceramic media and four AISI 1018 strip steel samples. Solutions consisting of large amounts of oxalic acid and small amounts of sodium oxalate were not soluble, therefore often not used for testing. Precipitation of oxalic acid monohydrate occurred.

The average roughness of these samples was between 15 and 20 μ-in. at the beginning of the test. Every half hour, a strip steel sample was taken out, rinsed with demineralized water and dried with a paper towel. The roughness was measured and the weight loss determined to calculate the material removed in μ-in. After two hours of acid treatment time, the test was terminated.

Figures 6 and 7 show the material removal and the average roughness after two hours of acid treatment versus the pH of these acid treatment solutions. The material removed after two hours of acid treatment goes through a minimum at approximately pH 1.9 (Fig. 6). It is assumed that the material removal decreases with increasing pH after reaching a maximum at approximately pH 3.5. The average roughness after two hours of acid treatment (Fig. 7) is around 3 μ-in. in the pH range of 1.1 to 3.2. At a higher pH, the roughness does not decrease as much as at the lower pH ranges.

Phase III: Oxalic acid / potassium oxalate systems

For the third part of this test series, different acid treatment solutions, mostly based on oxalic acid and potassium oxalate were tested. It is known that in general potassium salts have an even higher solubility in water than sodium salts. The objective was to see what would happen if the concentration of oxalate and the pH were increased. Potassium oxalate solutions were produced by adding potassium bicarbonate to an equivalent amount of oxalic acid. Unfortunately the carbon dioxide produced was hard to remove by cooking the solution because the expected pH was above 7. This resulted in potassium oxalate solutions with more or less dissolved carbon dioxide and a higher or lower solution pH. This is reason why the 0.5 and 0.7M potassium oxalate solutions have a pH span in Tables 4 and 5 and in Figs. 8 and 9.

Forty-seven milliliters of acid treatment solution made from various amounts of 0.7M oxalic acid and 0.7M sodium oxalate solution were added to 3745 g of white lens-shaped ceramic media and four AISI 1018 strip steel samples. Solutions consisting of large amounts of oxalic acid and small amounts of potassium oxalate were not stable, and precipitation occurred. Therefore these solutions were not used for testing. More information about the 0.7M solutions used is given in Table 4.

The average roughness of these samples was between 15 and 20 μ-in. at the beginning of the test. Every half hour, a strip steel sample was removed, rinsed with demineralized water and dried with a paper towel. The roughness was measured and the weight loss determined to calculate the material removed in μ-in. After two hours of acid treatment time, the test was terminated.

Because of the solubility issues mentioned above, the concentration of the oxalate solutions were lowered to 0.5M, then to 0.4M and finally to 0.3M. The 0.3M oxalic acid solution was soluble with the 0.3M potassium oxalate solution at every ratio without forming a precipitate. Information about the solutions is given in Tables 5 to 7. All results concerning the third series of tests are documented in Figs. 8 and 9.

In all cases studied between the pH of 0.6 and 3.0, the material removal after two hours of acid treatment was between 8 and 23 μ-in., and the average roughness was in a small band of 2.31 to 3.39 μ-in. If the starting pH was higher than 3.0, the material removal went through a maximum at around pH 3.9, and the average roughness (after two hours of acid treatment) increased with the pH.

Phase IV: Oxalic acid / potassium ammonium oxalate systems

For the fourth part of this test series, different acid treatment solutions based on oxalic acid and potassium ammonium oxalate (Table 8) were tested. The reason for this test series was to determine if by adding ammonium oxalate, the solubility and concentration of oxalate anion coming from oxalic acid and oxalate salt, could be increased. Ultimately this did not happen, but the results were worth the effort. Forty-seven milliliters of acid treatment solution made from various amounts of 0.4M oxalic acid and 0.4M potassium ammonium oxalate solution were added to 3745 g of white lens-shaped ceramic media and four AISI 1018 strip steel samples. Solutions consisting of large amounts of 0.4M oxalic acid and small amounts of 0.4M potassium ammonium oxalate were not stable, therefore not used for testing. Precipitation of oxalic acid monohydrate occurred.

The average roughness of these samples was between 15 and 20 μ-in. at the beginning of the test. Every half hour, a strip steel sample was taken out, rinsed with demineralized water and dried with a paper towel. The roughness was measured and the weight loss determined to calculate the material removed in μ-in. After two hours of acid treatment time the test was terminated.

The curves for the material removed (Fig. 10) and the average roughness (Fig. 11) versus starting pH fit well into the charts from the previous test series.

A closer look at the material removal rates will give additional important information about the CAVSF process. Figures 12 to 17 show the material removal during acid time for the last three test series.

The material removal rates for all solutions with sodium or potassium oxalate and a relatively small amount of oxalic acid are extremely high when compared to the other values in the test series. The maximum material removal rate of a test series at the beginning of the process increases with the oxalate concentration (the sum of the oxalic acid and oxalate salt concentration). It is 25 μ-in./hr for the 0.2M oxalate concentration (Fig. 12) and 125 μ-in./hr for 0.7M oxalate solutions (Fig. 16).

Another observation can be made if Figs. 14 and 17 are compared. Both graphs show a plot for 0.4M oxalate. Figure 14 shows the mixtures of oxalic acid and potassium oxalate and Figure 17 the mixture of oxalic acid and potassium ammonium oxalate. Obviously the ammonium cation increases the removal rate at the beginning of the process with 0.4M salt solution (no acid). With some acid in the solution at 0.1M (COOH)2 and 0.3M KNH4(COO)2, the removed material increased linearly with time for two hours instead of leveling out as observed with 0.1M (COOH)2 and 0.3M K2(COO)2.

Phase V: Oxalic acid / ammonium oxalate systems

The influence that solution acidity has on material removal and end roughness of the CAVSF process was investigated with 0.4M oxalate solutions consisting of various amounts of 0.4M oxalic acid and 0.4M ammonium oxalate solution. Each mixture of these solutions was completely soluble and did not form precipitates at room temperature. Ammonium oxalate was chosen because of the high solubility in water the low pH of the salt solution and the ability of the ammonium molecule to build week complexes with the iron ions, which could have a visible effect on the process. No other investigated 0.4M oxalate solution showed this high solubility, not sodium, not potassium and not potassium ammonium oxalate.

Table 9 describes the composition and the pH of the solutions at the beginning of the test and also the end results, i.e., the material removed and the average roughness at the end of the two-hour acid treatment.

Figures 18 and 19 show the material removal and the average roughness respectively versus acid treatment time for the different solutions. The higher the ammonium oxalate concentration in these solutions the lower the oxalic acid concentration and the higher the pH and the higher the initial material removal rate in Fig. 18. After a one-hour acid treatment with 0.4M ammonium oxalate no more material is removed. The passive layer is too strong for the weakened acid and/or the rubbing media. If the oxalic acid concentration in these solutions is lower than 0.075M, the average roughness after two hours is still high or increased after reaching a low point (Fig. 19).

Figures 20 and 21 show the material removal and the average roughness after two hours of acid treatment versus the pH of the acid treatment solutions. The material that was removed after two hours of acid treatment goes through a minimum at pH 2 and a maximum at pH 3.8 (Fig. 20). In all the tests between pH 0.9 and 3, the material removal after two hours of treatment was between 12 and 23 μ-in. The average roughness after two hours of acid treatment increased slightly from 2.3 to 3.0 μ-in. from pH 0.9 to pH 3. At pH 3, the two-hour average roughness increased sharply with the pH (Fig. 21).

Summary

The chemically accelerated vibratory surface finishing (CAVSF) process with different oxalic acid-based solutions was studied using strip steel samples AISI 1018 in a 0.28 m diameter vibratory bowl. The effects of chemical composition and concentration on the process were examined. The average end roughness and the material removal reached with the process were charted versus the pH of the different mixtures. The best results concerning average end roughness gave oxalic acid solutions with a pH around 1. At pH values higher than 3, the average end roughness first increased rapidly with pH. At a pH around 2, the material removal versus pH curve reached a minimum and at a pH around 3.8 a maximum.

This project was sponsored by the Defense Technical Information Center. The work was made possible by the contractual relationship between AMMTIAC and DoD to research and analysis of advanced materials. This includes US Army Benét Laboratories with US Army ARDEC (Armament Research, Development and Engineering Center).

US Army Benét Laboratories, Alion Science and Technology and the Engineered Surfaces Center of the University of North Dakota were working together in improving life and performance of materials used for weapons systems.

Dr. Juergen Fischer is a Chemical Engineer with significant experience in both Electrochemistry and Chemistry. He is currently employed as a Senior Engineer with the Engineered Surfaces Center (ESC) within the School of Engineering and Mines at the University of North Dakota. Dr. Fischer started his career as a Chemistry laboratory assistant for BASF in Ludwigshafen, Germany. He then pursued his educational career earning his B.S. in Chemical Engineering at the Engineers School in Darmstadt, Germany, as well as his M.S. and Ph.D. in Chemical Engineering at the Technical University of Darmstadt in Darmstadt, Germany. Dr. Fischer was employed for eight years as an R&D Manager and Chemical Engineer at AlumiPlate, Inc. in Coon Rapids, Minnesota. Highlights of his career include R&D of advanced processes concerning water electrolysis, acid recovery and aluminum plating and chemical engineering for a water electrolysis plant, four aluminum plating plants, an acid recovery plant, and two recycling systems including vacuum rectification and evaporation. Dr. Fischer holds five patents / patent applications.

Mr. Douglas Larson, M.S. is currently employed as a Senior Research Engineer at the Advanced Engineered Materials Center at the University of North Dakota in Grand Forks, ND. He contributed to the physical establishment of the Center and has conducted metal fatigue research related to the surface treatment of superfinishing, corrosion studies of friction stir welds, establishing friction stir welding capabilities at the center. Mr. Larson received both a Bachelor of Science and a Master of Science degree in Mechanical Engineering from the University of North Dakota with study emphasis in manufacturing engineering. His industrial experience includes design and manufacturing engineering of construction machinery and wood processing equipment and as an electric utility project engineer. His administrative experience includes being a project director, equipment maintenance supervisor, safety coordinator and computer system administrator. He is currently establishing friction stir weld modeling capabilities at the center and is performing further friction stir weld research.

Dr. Samar J. Kalita is a Materials Science Researcher with experience in laser materials processing, nanocrystalline ceramics, electrochemistry, biomaterials, rapid prototyping and engineering education. Dr. Kalita has a Ph.D. in Materials Science from Washington State University and a B.E. in Metallurgical Engineering from the National Institute of Technology-Tiruchirappalli, India. At present, he is a Senior Engineer of the Advanced Engineered Materials Center and a Graduate Faculty (adjunct) of Mechanical Engineering Department at the University of North Dakota. Previously, he worked as an Assistant Professor of Materials Science and Engineering at the University of Central Florida for 5½ years, and as a Metallurgist in an automobile company for two years. He has authored 19 journal articles, two book chapters, 12 conference papers and 47 conference presentations. He supervised six M.S. theses and served in 11 Ph.D. and five additional M.S. committees. He refereed articles for 12 journals, and participated in several conferences as presenter, invited speaker, session chair, reviewer and committee member. He has also received several academic honors and awards.

Dr. D. Bryce Mitton is Director of the Engineered Surfaces Center within the School of Engineering and Mines at the University of North Dakota. Dr. Mitton was previously employed as a Research Associate and was the Senior Research staff member within the H. H. Uhlig Corrosion Laboratory at MIT, where he was Co- PI on a variety of projects. In 1979, he graduated "Cambrian Scholar" from Cambrian College of Applied Arts and Technology in Canada and subsequently was involved in an investigation of corrosion associated with high level nuclear waste containment at the Atomic Energy of Canada Limited (AECL). Dr. Mitton received a M.Sc. in Corrosion Science and Engineering in 1984, was awarded a Part-Time Teaching Assistantship in 1985, and a Ph.D. in Corrosion Science and Engineering in 1990 from The University of Manchester Institute of Science and Technology (UMIST) in England. He has diverse research experience, more than 70 publications, including book chapters and invited papers, and has both consulting and teaching experience.

Benjamin Hoiland is a Research Engineer at Alion Science & Technology, and has been with the company since 2005. Mr. Hoiland now leads the group of research engineers at the Surface Engineering Center. The center’s focus is to increase the life and improve the reliability of Army equipment housings, frames, chassis, other parts with complex geometries, specific materials of construction and load bearing elements through the application of advanced production process technology to the fabrication of replacement or new parts. Mr. Hoiland received a Bachelor of Science degree in Mechanical Engineering from the University of North Dakota.

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